Magnetic head-based systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic tape drive system, a magnetic tape containing a multiplicity of laterally positioned data tracks that extend along the length of the tape is drawn across a magnetic read/write transducer, referred to as a magnetic tape head. The magnetic tape heads can record and read data along the length of the magnetic tape surface as relative movement occurs between the heads and the tape.
In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while a magnetic head “flies” slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor.
Magnetoresistive (MR) sensors are particularly useful as read elements in magnetic heads, used in the data storage industry for high data recording densities. Three examples of MR materials used in the storage industry are anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR). An MR sensor is one whose resistance is changed by a magnetic field. MR, e.g., AMR, GMR and TMR, sensors are deposited as small and thin multi-layered sheet resistors on a structural substrate. The sheet resistors can be coupled to external devices by contact to metal pads which are electrically connected to the sheet resistors. MR sensors provide a high output signal which is not directly related to the head velocity as in the case of inductive read heads.
To achieve the high areal densities required by the data storage industry, the sensors are made with commensurately small dimensions. The smaller the dimensions, the more sensitive the thin film resistors become to damage from spurious current and voltage spikes.
A major problem that is encountered during manufacturing, handling and use of MR sheet resistors as magnetic recording transducers is the buildup of electrostatic charges on the various components of a head or other objects which come into contact with the sensors, particularly sensors of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be externally produced and accumulate on instruments used by persons performing head manufacturing or testing function. These static charges may be discharged through the head, causing physical and/or magnetic damage to the sensors.
As described above, when a head is exposed to voltage or current inputs which are larger than that intended under normal operating conditions, the sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for high recording densities for magnetic tape media (on the order of 25 MBytes/cm2) are patterned as resistive sheets of MR and accompanying materials, and typically have a combined thickness for the sensor sheets on the order of 500 Angstroms (Å) with a width of a few microns (μm) and a height on the order of 1 μm. Sensors used in extant disk drives are even smaller. Discharge currents of tens of milliamps through such a small structure can cause severe damage or complete destruction of the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, oxidation of materials at the tape bearing surface (TBS), generation of shorts via electrical breakdown, and milder forms of magnetic or physical damage in which the head performance may be degraded. Short time current or voltage pulses which cause extensive physical damage to a sensor are termed electrostatic discharge (ESD) pulses.
For example, in current full span tape head modules, an insulative adhesive layer is used for bonding a closure to the thin film stack portion of the device chip, which contains the magnetic read, write and servo elements. Thus, the closure and chip are electrically isolated. Usually the chip itself is connected to ground or other reference voltage in the tape drive, leaving the closure floating. Tape running can then lead to significant closure charging resulting in electrical potential difference between substrate and closure of up to 10s of volts. This is known to contribute to such problems as shorting, alumina pitting, accumulation of adherent debris and others. Also, ESD can cause shorting and result in damage of sensors and read-write elements. To prevent closure charging and ESD, an adhesive should be selected that has electrostatic dissipative properties, thus effectively reducing the chance of electrostatic discharge.
There are many applications where devices are bonded together that require a specific adhesive to satisfy bond strength requirements for the particular materials, as well as satisfying other mechanical properties. For example, in current full span tape head modules, an insulative adhesive layer is used for bonding the closure to the thin film stack portion of the device chip, which contains the magnetic read, magnetic write, and magnetic servo transducers. However, this electrically isolates the various portions of the head.
In some cases, an electrical connection is required to connect the devices together. In other cases, the electrical connections require an ESD dissipative connection (about 104 to about 1011 ohms resistance). An example of such use would be the attachment of conductive ceramics used in tape heads. In tape heads, wafers made of conductive ceramics are used to deposit the read and write elements used to read and write data onto magnetic tapes. In some cases, two or more conductive ceramics are bonded together with a strong adhesive and are then cut and polished to form a smooth TBS. The tape head components are usually not connected electrically. When worn against tape, these conductive materials at the TBS can be charged up due to tribocharging with the tape. Also, the metal material forming the read-write materials are also in contact with the tape and can have a voltage either due to tribocharging of the tape or because of passing electrical charges used in the device operation.
It has been found that it is beneficial to set the potential of the head components to specific values and in some cases to keep all components at the same electrical potential. One solution is to use a silver paint to connect the closure and the substrate together, which is used in IBM's LTO tape drives. While this usually works, there are reliability issues involving the silver paint flaking off as the paint dries. Also, the application of the silver paint to the head components is an additional step in the processing of the head.
Another solution is using a conductive element for clamping the head components at the same electrical potential. This also has reliability issues in that the material used for the bond could get into the elements and cause a short. Also, this adds another processing step in manufacturing the head.